I have used many thermostats and timers, in the old days boilers simply turned on or off, and they worked of sorts, but today as already said the condensation part of the boiler needs the return water temperature to be below a set figure so they no longer turn on and off but use the return water temperature to adjust flame size so in theory as each one of the TRV (thermostatic radiator valve) turns off the system adjusts to suit.
The problem is the boiler can be too clever and unless the lock shield valves and TRV's are set correct the whole system can fall over.
There is a system called Evohome which is designed to work with special TRV's which monitor the temperature and time for every room so although the boiler is central each room is independently controlled, however it is expensive.
So what we try to do is use cheaper options to get what we want, at least I have. However they tend to fight each other for control, so it takes a lot of tweaking to get it to run well.
With a system left running 24/7 the TRV will look after its self. The problem is after the system is turned off. So what is all important is the lock shield valve. If they are left wide open then what happens is all the hot water goes to one radiator, as the room warms up, the radiator switches off and it all goes to next radiator, and so room by room the house is warmed up again. The returning hot water switches the boiler down to minimum setting and the house holder thinks the boiler is not big enough. Reducing the flow in each radiator means they all get hot water together and no hot water returns to boiler until all radiators are hot. Once the bypass valve opens as every TRV closes then the boiler will shut down, it will shut down for a time which varies, and then it tries firing up and testing the return water temperature, if still high it increase the time before next try, if not it decreases the time before next try, this is the anti-cycle software working.
However if the supply to the radiator is cut by a timer rather than temperature then the boiler will think the house is hot enough and increase the re-try time to a much too long of a time, so once the timer switches on, there is no hot water being pumped around, so there is a delay before the boiler starts.
This delay is compounded by the thermostat delay as well, the result is you come home to a cold house.
When I wrote the first reply I was unaware of the thermostat delay. I have a hard wired thermostat and timer at home, so at 6 am the temperature set changes from 14°C to 20°C and boiler starts straight away. However mothers house has a wireless thermostat and from time set it takes 15 minutes some times before the two have shaken hands and the boiler has started.
Today for some unknown reason my mother sat in her wheel chair at the back door with it wide open, the house temperature went down, and 1/2 hour latter the boiler cut in to re-heat the house, lucky I have a stand alone fire I could use while I waited for the thermostat to talk to the base unit. But there is no data with the thermostat timers to tell you what delay they have.
So unless you can afford EvoHome then best is to use timer built into boiler, and not use any exterior add on devices. Learn from my error.